This invention is in the field of silicoaluminophosphate (SAPO) membranes, in particular SAPO-34 membranes prepared on a porous support. The invention provides supported SAPO-34 membranes as well as methods for making and using them.
SAPOs are largely composed of Si, Al, P and O and can have a three-dimensional microporous crystal framework structure of PO2+, AlO2− and SiO2 tetrahedral units. The cages, channels and cavities created by the crystal framework can permit separation of mixtures of molecules based on their effective sizes.
SAPO crystals can be synthesized by hydrothermal crystallization from a reaction mixture containing reactive sources of silica, alumina, and phosphate, and an organic templating agent. Lok et al. (U.S. Pat. No. 4,440,871) report gel compositions and procedures for forming several types of SAPO crystals, including SAPO-5, SAPO-11, SAPO-16, SAPO-17, SAPO-20, SAPO-31, SAPO-34, SAPO-35, SAPO-37, SAPO-40, SAPO 41, SAPO-42, and SAPO-44 crystals. Lok et al. do not appear to disclose formation of SAPO membranes. Prakash and Unnikrishnan report gel compositions and procedures for forming SAPO-34 crystals. (Prakash, A. M. and Unnikrishnan, S., J. Chem. Sc. Faraday Trans., 1994, 90(15), 2291-2296). In several of Prakash and Unnikrishnan's reported procedures, the gel was aged for 24 hours at 27° C. (300 K). Prakash and Unnikrishnan do not appear to disclose formation of SAPO-34 membranes.
SAPO membranes have been proposed for use in gas separations. For these applications, an important parameter is the separation selectivity. For two gas components i and j, a separation selectivity Si/j greater than one implies that the membrane is selectively permeable to component i. If a feedstream containing both components is applied to one side of the membrane, the permeate stream exiting the other side of the membrane will be enriched in component i and depleted in component j. The greater the separation selectivity, the greater the enrichment of the permeate stream in component i.
Barri et al. report supported zeolite membranes (U.S. Pat. No. 5,567,664) and methods for the production of zeolite membranes on porous supports (U.S. Pat. No. 5,362,522). Barri et al. state that any type of zeolite-type material may be used, including silicoaluminophosphates.
SAPO-34 membranes on porous supports have been reported in the scientific literature. Lixiong et al. (Stud. Surf. Sci. Catl., 1997, 105, p 2211) reported synthesis of a SAPO-34 membrane on one side of a porous α-Al2O3 disk by immersing the substrate surface in a hydrogel and heating the substrate and gel. Lixiong et al. reported single gas permeances for H2, N2, CO2, and n-C4H10. Poshuta et al. (Ind. Eng. Chem. Res., 1998, 37, 3924-3929; AlChE Journal, 2000, 46(4), 779-789) reported hydrothermal synthesis of SAPO-34 membranes on the inside surface of asymmetric, porous α-Al2O3 tubes. Poshuta et al. (supra) reported single gas and mixture permeances and ideal and mixture selectivities for several gases, including CO2 and CH4. The CO2/CH4 selectivities reported for a 50/50 CO2/CH4 mixture at 300 K were between 14 and 36 for a feed pressure of 270 kPa and a pressure drop of 138 kPa (Poshusta et al., AlChE Journal, 2000, 46(4), pp 779-789). The CO2/CH4 selectivity was attributed to both competitive absorption (at lower temperatures) and differences in diffusivity. Li et al. reported an average CO2/CH4 selectivity of 76+/−19 for a 50/50 CO2/CH4 mixture at 295 K with a feed pressure of 222 kPa and a pressure drop of 138 kPa. The average CO2 permeance was (2.3+/−0.2)×10−7 mol/(m2sPa) and the average CH4 permeance was (3.1+/−0.8)×10−9 mol/(m2sPa). (Li, S. et al, Ind. Eng. Chem. Res. 2005, 44, 3220-3228. U.S. Patent Application Publication 2005-0204916-A1 to Li et al. reports CO2/CH4 separation selectivities of 67-93 for a 50/50 CO2/CH4 mixture at 297 K with a feed pressure of 222 kPa and a pressure drop of 138 kPa.
Several U.S. Patents report processes for the manufacture of molecular sieve layers on a support which involve depositing or forming molecular sieve crystals on the support prior to an in situ synthesis step. U.S. Pat. No. 6,090,289 to Verduijn et al. reports a process which involves forming an intermediate layer by applying molecular sieve crystals to the support or forming such crystals on the support then contacting the resulting coated support with a molecular sieve synthesis mixture and subjecting the mixture to hydrothermal treatment in order to deposit an upper layer comprising a crystalline molecular sieve of crystals having at least one dimension greater than the dimensions of the crystals of the intermediate layer. U.S. Pat. No. 6,177,373 to Sterte et al. reports a process which involves depositing on a substrate a monolayer comprising molecular sieve monocrystals which are capable of nucleating the growth of a molecular sieve film, forming a molecular sieve synthesis solution, contacting the monolayer and the synthesis solution and hydrothermally growing molecular sieve to form a molecular sieve film on the substrate. U.S. Pat. No. 5,871,650 to Lai et al. reports a process for preparing a zeolite membrane exhibiting a columnar cross-sectional morphology.
There remains a need in the art for improved methods for making SAPO membranes, in particular SAPO membranes with improved separation selectivities.